In many glass processing systems, horizontal roller conveyors are utilized to convey heated glass sheets during processing. For example, many conventional systems designed to temper and/or heat strengthen glass sheets utilize horizontal roller conveyors for conveying glass sheets first into a furnace for heating and then out of the furnace for tempering or heat strengthening. Within the furnace, it is conventional to utilize ceramic or steel rolls which are capable of withstanding a heated ambient on the order of about 1200 to 1250 degrees Fahrenheit (i.e. about 650 to 675 degrees Centigrade) for a prolonged period.
In glass sheet tempering systems utilizing roller conveyors, conveyor rolls at the exit location of the furnace receive the heated glass sheets for conveyance through a quench unit in which blasts of cooling gas are supplied to the oppositely facing glass surfaces to provide tempering of the sheets. Such tempering places the glass surfaces in compression in order to improve the mechancal properties of the glass sheets by providing greater resistance to breakage and the characteristic of breaking into small dull pieces rather than into sharp slivers.
In heat strengthening of glass sheets, heated glass sheets are conveyed out of the furnace for cooling at a faster rate than takes place with annealing such that there is a certain degree of compression at the glass surfaces but much less compression than is the case with tempered glass sheets. Such heat strengthened glass sheets have utility in laminated panels due to their ability to deform without breaking upon differential thermal expansion of the layers of these panels. Heat strengthening of glass sheets can be performed on tempering systems by conveying the heated glass sheets from the furnace through the quench unit without supplying any quenching gas or only a very small flow such that the glass surfaces are not placed in a very high state of compression upon ultimate cooling.
Glass sheets must be heated to a temperature range of about 1100 to 1200 degrees Fahrenheit (i.e. about 600 to 650 degrees Centigrade) in order to perform effective tempering or heat strengthening upon ultimate cooling. At temperatures substantially lower than this range, the glass is too viscous to permit the surfaces to be placed in compression during tempering or heat strengthening. At temperatures substantially higher than this range, the glass is too fluid to allow the tempering or heat strengthening to be performed while maintaining planarity.
U.S. Pat. Nos. 3,806,312; 3,934,970; 3,947,242; and 3,994,711 disclose glass tempering systems of the horizontal roller conveyor type for conveying glass sheets in a horizontal direction during a tempering process. Rolls in both the furnace and quench unit of each of the tempering systems disclosed by these patents are supported at their opposite ends and frictionally driven by continuous drive loops. Glass sheets supported on the upper sides of the rolls are conveyed therealong first through the furnace where heating takes place and then through the quench unit where quenching gas supplied by upper and lower blastheads provides rapid cooling of the heated glass to perform the tempering.
One conventional type of conveyor roll for quench units of horizontal roller conveyor type glass sheet tempering systems includes a central steel shaft and a flexible metallic component embodied by an all metal wire or a metallic helical spring wrapped about the shaft in a helical configuration. Such helical metallic components reduce the extent of contact with the soft heated glass sheets and also increase the exposed lower glass sheet surface area which is subjected to quenching gas as tempering is performed. In addition, the helical metallic components prevent lifting of conveyed glass sheets during tempering by facilitating the escape of spent quenching gas downwardly after impinging with the lower glass surface. Since the metallic components remain rigid at the temperature involved, the softened glass sheet can be chill cracked or marked by the helical metallic component if all of the furnace, quench unit, and and conveying parameters are not properly selected.
Other types of conveyor rolls for horizontal roller conveyor type systems for tempering and/or heat strengthening glass sheets include annular silicone rubber members received by a central steel shaft and asbestos rope wrapped helically around a central steel shaft. One problem with these types of quench rolls is that the occasional glass sheet that breaks cuts the annular silicone rubber member or the asbestos rope and the roll must then be repaired before further use. Also, the silicone rubber can leave a thin layer on the heated glass sheet if the glass temperature is too high. In attempt to overcome these problems, prior quench rolls of glass tempering systems have been provided with helical components including a central core and an outer nonmetallic covering which is a synthetic organic polymer such as silicone rubber or Teflon. The central core utilized includes metallic wire strands so as to have sufficient strength to resist cutting by broken glass.
Quench rolls for tempering relatively wide glass sheets have also included a central shaft along which glass fiber conveying discs are positioned to carry the glass during tempering. Such a quench roll construction increases the escape area for spent quenching gas supplied by a lower blasthead.